Conventional hearing aids suffer from several shortcomings. It is difficult if not impossible with conventional hearing aids to provide a frequency-gain characteristic that is ideal for each individual user. The acoustic coupling between the hearing aid receiver and the eardrum also introduces changes in the frequency-gain characteristic that is usually deleterious to both speech intelligibility and overall sound quality. For many patients, the optimum frequency-gain characteristic varies as a function of the level of the speech signal reaching the hearing aid. In order to protect patients from uncomfortably or dangerously loud signals, it is also necessary to limit the maximum acoustic power output of the hearing aid in some way. The methods used to limit acoustic power output of hearing aids typically introduce deleterious distortions to the amplified speech signal.
Another common problem is that of acoustic feedback. Even in the best designed hearing aids, not all of the amplified acoustic signal is delivered to the eardrum. A small proportion of the amplified acoustic signal leaks back to the hearing aid microphone forming an acoustic feedback loop. If the gain of the hearing aid is sufficiently high, this acoustic feedback will cause a self-generating oscillation to occur, resulting in an unwanted and highly unpleasant whistling sound. These acoustic oscillations prevent the hearing aid from being used. Methods of acoustic feedback control that are typically used include a tighter acoustic seal between the earmold and the walls of the ear canal so as to reduce acoustic leakage, placing the microphone at some distance from the hearing aid receiver, e.g. on the opposite ear, or simply reducing the gain of the hearing aid. None of these methods provides a satisfactory solution for high-gain hearing aids.
One of the most common complaints of hearing aid users is that background noise is particularly damaging to the understanding of speech. Methods currently used to reduce background noise in hearing aids employ filtering techniques in which the frequency regions containing high noise levels are eliminated.
Another common problem is that of room reverberation produced by acoustic reflections off the walls, ceiling, floor, and other surfaces in a room. A small amount of reverberation is beneficial but too much reverberation will make a room sound hollow or echoic and will interfere with both the quality and the intelligibility of speech.
Systems have been proposed heretofore utilizing computers for testing the hearing of patients and generating programming for a programmable hearing aid, as disclosed in "Computer Application in Audiology and Rehabilitation of the Hearing Impaired" by Harry Levitt, Journal of Communication Disorders 13 (1980), pages 471-481, and in the U.S. Pat. Nos. 4,187,413 to Moser, 4,489,610 to Slavin, and 4,548,082 to Engebretson et al., for example. None of these systems, however, affords a satisfactory solution for the problems of acoustic feedback, background noise, room reverberation and changes in the optimum frequency-gain characteristic resulting from variations in the level of the speech signal reaching the hearing aid.
It is an object of the invention, accordingly, to provide a new and improved hearing aid system that is free of the above-noted deficiencies of the prior art.
Another object of the invention is to provide new and improved hearing aid apparatus of the above character which is capable of automatically adjusting to an optimum set of parameter values as the speech level and type of background noise change.
A further object of the invention is to provide new and improved hearing aid apparatus of the above character in which acoustic feedback is substantially reduced.
Still another object of the invention is to provide new and improved hearing aid apparatus that is capable of effective noise and reverberation suppression and acoustic feedback reduction while maintaining optimum hearing characteristics as the speech and noise levels vary.